Semiconductor substrates and devices are becoming smaller and more dense with the evolution of new technologies. However, increases in circuit density produce a corresponding increase in overall manufacturing problems. These manufacturing problems must however be kept to a minimum in order for the semiconductor manufacturer to remain competitive. The semiconductor manufacturers are therefore constantly being challenged to improve the quality of their products by identifying and eliminating defects which produce defective parts or components. Whereas significant improvements are being made to eliminate systematic defects by reducing process variability, process improvements alone are not sufficient to eliminate all the random defects which effect both yield and reliability. Historically, screening techniques have been employed to improve product failure rates to acceptable levels by culling out many of these random defects.
In their desire to improve their products, the semiconductor manufacturers are constantly finding new ways and new techniques to improve or provide new products. It has been found that for some applications, one could make a ceramic carrier or substrate (hereafter just substrate) having a cavity and then have the semiconductor chip placed inside the cavity and secured to the substrate. These semiconductor substrates are often referred to as modules and could be made from a single or a multi-layer ceramic sheet or green sheet forming a single layer ceramic module or a plurality of ceramic layers forming an MLC (multilayer ceramic) module.
While the remaining discussion will be directed to MLC modules, it should be understood that the teachings of the present invention can be equally applicable to single layer modules.
MLC modules having single or multiple cavities are normally used in the electronic industry to package high performance integrated circuits or chips (hereafter just chips). These high performance chips have a large number of external inputs/outputs (called I/Os), such as pads or solder balls, to name a few, and these chips have a very high power dissipation. In order to accommodate such high performance chips, the MLC module also has to provide a high number of external I/Os, such as pads, pins, solder balls, to name a few, and also be able to handle the very high power dissipation that is being generated both from the module as well as the chip. Furthermore, there may also be wire bond pads on the shelves within the cavity.
The single or multiple cavities in an MLC substrate are normally formed by laminating the green sheets during the lamination process typically with the aid of a plug or insert, such as, disclosed in IBM Technical Disclosure Bulletin, "Fixture For Fabricating Complex Substrate Design From Green Sheet Ceramics", Phillips, Vol. 16, No. 11, Page 3559 (April 1974), the disclosure of which is incorporated by reference herein, this contoured laminating fixture with projections that coincide with the recesses in the ceramic substrate, in turn prevents the collapse or deformation of the recesses in the ceramic green sheet during lamination. This method of producing single or multiple cavities requires machining of the inserts with high precision and with high level of surface finish.
Inherently, the cost of such plugs or inserts is very high compared to the cost of the substrate. Additionally, these inserts or plugs do not provide the flexibility of using the same inserts for cavities having various shapes and sizes. Furthermore, placing these inserts in the cavities and then subsequently removing them is an expensive process. It has also been observed that subsequent removing of these inserts, in some cases, has lead to the delamination of the ceramic green sheets or other damage to the green ceramic body. Another drawback with these solid inserts is the need to clean them prior to every use to avoid the paste pull-outs or damage to the green ceramic layers or pads. Even with cleaning of these inserts, paste pull-out often occurs due to the lack of an effective release layer.
Another method of producing these single or multiple cavities in the MLC substrate would be to machine the cavities after the green sheets have been stacked and laminated, but this would not be a cost effective way of producing parts in a high volume manufacturing operation.
It is also possible to form cavities in the MLC substrate with no inserts. This could be done for cases where the lamination conditions are such that there is no resulting deformation in the green ceramic body. In these cases, typically, the lamination pressures are very low and the green sheet formulation is such that the dimensional control of products is achieved by altering the sintering process. However, in a high volume manufacturing operation, tailoring the green sheet formulation and developing a sintering cycle for every product would be cost prohibitive and time consuming. Besides, this approach typically needs an adhesive between layers and multiple lamination steps to achieve the end result. Thus, some of the problems associated with this low pressure lamination process are that no process window for dimensional control is available for the sintered body. Delamination of the ceramic green sheets could happen in sintering due to the removal of the adhesive and the density gradients in the starting structure that are normally present could result in poor substrate dimensional control. Furthermore, there could be substantial increase in stacking and lamination cost and limitation in metal loading on the green sheets to have effective green sheet bonding.
The prior art has approached this problem in other ways as well.
U.S. Pat. No. 4,680,075 (McNeal et al.) and U.S. Pat. No. 4,636,275 (Norell), the disclosures of which are incorporated by reference herein, disclose laminating with a preformed plugs with thermoplastic material such as polyvinyl chloride, polyolefin or a poly carbonate. This plug goes through a plastic deformation at lamination temperature and pressure to fill the cavity.
U.S. Pat. No. 4,737,208 (Bloechle et al.), the disclosure of which is incorporated by reference herein, discloses a method of laminating a printed wiring board (non-ceramic) by utilizing a metallic template (which relieves some of the stresses on the corners of the layers), a fluropolymer (such as TFE) release layer and a putty-like conformal material (for example, flowable and not polymerized rubber) which fills the cavity.
U.S. Pat. No. 5,538,582 (Natarajan, et al.), assigned to International Business Machines Corporation, Armonk, N.Y., USA, and the disclosure of which is incorporated herein by reference, discloses a method for forming cavities in multilayer ceramics without using an insert.
U.S. Pat. No. 5,759,320 (Natarajan, et al.) and No. 5,788,808 (Natarajan, et al.), assigned to International Business Machines Corporation, Armonk, N.Y., USA, and the disclosures of which is incorporated herein by reference, discloses a method and apparatus for forming cavities in multilayer ceramics without using at least one compressive pad.
In every cavity formation technique, it is essential that the material set is chosen such that a cavity profile with sharp corners and flat wire bonding shelves is achieved. When improper plug and/or cavity-fill materials are chosen, the resulting cavity turns out to have rounded edges and corners and/or sloped wire bond shelves and/or paste pullouts, etc.
The present invention, however, solves these and other problems and the result is a ceramic cavity substrate with well-defined features as more fully described in the following description taken along with the accompanying drawings.